i think you mean glass pack muf. Justus becks, thats a muf. shell that has matted fiberglass packed in it and most of the time cheap and loud. after a while the fiberglass will burn out of it and youll have a stright through muf. most states dont give tickets for the type muf. but how much noise the car makes. i have run them on a lot of things, but got away from the noise a few yrs ago. if you like them thats all that counts.